1. Field of the Invention
This invention pertains to relief valves.
2. Description of the Prior Art
A relief valve in a liquid flow system is used as a bypass means for bleeding off a portion of the flowing liquid when, for example, one or more of the outlet passages of the flow system is closed to flow of the liquid. This often gives rise to a phenomenon known as relief valve "pressure droop", a detrimental drop in the pressure in the system as the bypass flow through the relief valve is reduced. In a conventional relief valve, small bypass flow rates therethrough correspond to small valve plug lift heights, corresponding to small spring deflections where a spring acts to urge the relief valve plug against its corresponding seat. In a conventional relief valve, the force required to lift the valve plug off its seat is determined solely by the compressive spring force. This force is opposed by the liquid pressure acting upwardly on a surface (hereinafter called the control surface) located above the liquid plenum associated with the valve, the control surface being connected with the surface upon which the spring force acts. The pressure at which the system operates is controlled by adjusting the initial compression of the spring. An equilibrium is established in which the spring deflection force equals the control surface pressure lifting force. If the flow rate through the relief valve is increased, the plug must be lifted higher off its seat to pass the increased flow at the same pressure. As the flow rate is reduced, back pressure throttled through the valve is also reduced; thus, a reduced pressure acts on the control surface and the upward force opposing the spring force is reduced. The spring then urges the valve downwardly, and a new, lower equilibrium plenum pressure is established corresponding to a smaller upward spring deflection. The accompanying undesirable decrease in plenum pressure as the flow rate is reduced is commonly known as "pressure droop".
U.S. Pat. No. 3,107,894 to Quinn teaches a valve plug and seat arrangement wherein the liquid flowing through the valve creates a force acting on the underside of the valve plug, tending to further open the valve. Quinn uses a frusto-conically shaped valve seat wall portion and a tapered valve plug to create a Venturi effect whereby the associated pressure differential assists in the rapid opening of the valve.
U.S. Pat. No. 2,622,613 to McNeal discloses a pressure control valve wherein the hydrokinetic force of the liquid, which is perpendicularly incident upon the control surface of the valve, holds the valve in an open position, and cylinders within the valve passageway create a dampening action to stabilize operation of the valve.
U.S. Pat. No. 2,804,089 and No. 2,755,815 to Siefferle and Erle, respectively, disclose pressure regulating valves with substantially different structures from the apparatus herein disclosed, such structures using the hydrokinetic forces of the flowing liquid to maintain a fairly constant discharge pressure in the valve over a range of flow rates.